A HID lamp is an arc-type lamp which produce light by forming an electric arc between electrodes housed in a fused alumina arc tube or transparent fused quartz filled with gas and metal salts. Once the arc is started using the gas, the metal salts are evaporated to form a plasma. The HID lamp converts input electrical energy into light energy by using input electrical energy to increase the energy in the plasma, producing light based on the collision of electrons and ions with neutral metal atoms.
FIG. 1 illustrates an example of a conventional HID lamp 10 having an outer ellipsoidal-shaped bulb 12 including an internal phosphor coating 14. The HID lamp 10 further includes an arc tube 16 connected to a support structure 18. A starting resistor 20 is connected with a starting electrode 22 for initiating the arc. A lower end of the bulb 12 is seated within a cap 24 for connecting to a power source. A ballast is used to operate the HID lamp 10.
In LED applications, light is generated more efficiently than in the HID applications. Light is generated when a conduction band electron re-combines with a hole in a valance band of the semiconductor. The semiconductor is created by doping a dielectric therein with donor (n-type) or acceptor (p-type) atoms. The LED is created by a sandwich of the n-type and p-type materials, such that the energy drop from conduction to valance band is equal to the energy of the light emitted (i.e., desired frequency or wavelength).
The LED is a structure that includes free electrons and holes such that when an electric field is applied across it, energy is transferred to the electrons and the holes more directly by increasing the drift velocity. Thus, more electrons can make the transition from the valence band to the conduction band, creating holes, and the electrons therefore recombine with holes generating a desired radiation.